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<div class="section" id="module-select">
<span id="select-waiting-for-i-o-completion"></span><h1>16.1. <a class="reference internal" href="#module-select" title="select: Wait for I/O completion on multiple streams."><tt class="xref py py-mod docutils literal"><span class="pre">select</span></tt></a> — Waiting for I/O completion<a class="headerlink" href="#module-select" title="Permalink to this headline">¶</a></h1>
<p>This module provides access to the <tt class="xref c c-func docutils literal"><span class="pre">select()</span></tt> and <tt class="xref c c-func docutils literal"><span class="pre">poll()</span></tt> functions
available in most operating systems, <tt class="xref c c-func docutils literal"><span class="pre">epoll()</span></tt> available on Linux 2.5+ and
<tt class="xref c c-func docutils literal"><span class="pre">kqueue()</span></tt> available on most BSD.
Note that on Windows, it only works for sockets; on other operating systems,
it also works for other file types (in particular, on Unix, it works on pipes).
It cannot be used on regular files to determine whether a file has grown since
it was last read.</p>
<p>The module defines the following:</p>
<dl class="exception">
<dt id="select.error">
<em class="property">exception </em><tt class="descclassname">select.</tt><tt class="descname">error</tt><a class="headerlink" href="#select.error" title="Permalink to this definition">¶</a></dt>
<dd><p>The exception raised when an error occurs. The accompanying value is a pair
containing the numeric error code from <tt class="xref c c-data docutils literal"><span class="pre">errno</span></tt> and the corresponding
string, as would be printed by the C function <tt class="xref c c-func docutils literal"><span class="pre">perror()</span></tt>.</p>
</dd></dl>
<dl class="function">
<dt id="select.epoll">
<tt class="descclassname">select.</tt><tt class="descname">epoll</tt><big>(</big><span class="optional">[</span><em>sizehint=-1</em><span class="optional">]</span><big>)</big><a class="headerlink" href="#select.epoll" title="Permalink to this definition">¶</a></dt>
<dd><p>(Only supported on Linux 2.5.44 and newer.) Returns an edge polling object,
which can be used as Edge or Level Triggered interface for I/O events; see
section <a class="reference internal" href="#epoll-objects"><em>Edge and Level Trigger Polling (epoll) Objects</em></a> below for the methods supported by epolling
objects.</p>
<p class="versionadded">
<span class="versionmodified">New in version 2.6.</span></p>
</dd></dl>
<dl class="function">
<dt id="select.poll">
<tt class="descclassname">select.</tt><tt class="descname">poll</tt><big>(</big><big>)</big><a class="headerlink" href="#select.poll" title="Permalink to this definition">¶</a></dt>
<dd><p>(Not supported by all operating systems.) Returns a polling object, which
supports registering and unregistering file descriptors, and then polling them
for I/O events; see section <a class="reference internal" href="#poll-objects"><em>Polling Objects</em></a> below for the methods supported
by polling objects.</p>
</dd></dl>
<dl class="function">
<dt id="select.kqueue">
<tt class="descclassname">select.</tt><tt class="descname">kqueue</tt><big>(</big><big>)</big><a class="headerlink" href="#select.kqueue" title="Permalink to this definition">¶</a></dt>
<dd><p>(Only supported on BSD.) Returns a kernel queue object; see section
<a class="reference internal" href="#kqueue-objects"><em>Kqueue Objects</em></a> below for the methods supported by kqueue objects.</p>
<p class="versionadded">
<span class="versionmodified">New in version 2.6.</span></p>
</dd></dl>
<dl class="function">
<dt id="select.kevent">
<tt class="descclassname">select.</tt><tt class="descname">kevent</tt><big>(</big><em>ident</em>, <em>filter=KQ_FILTER_READ</em>, <em>flags=KQ_EV_ADD</em>, <em>fflags=0</em>, <em>data=0</em>, <em>udata=0</em><big>)</big><a class="headerlink" href="#select.kevent" title="Permalink to this definition">¶</a></dt>
<dd><p>(Only supported on BSD.) Returns a kernel event object; see section
<a class="reference internal" href="#kevent-objects"><em>Kevent Objects</em></a> below for the methods supported by kevent objects.</p>
<p class="versionadded">
<span class="versionmodified">New in version 2.6.</span></p>
</dd></dl>
<dl class="function">
<dt id="select.select">
<tt class="descclassname">select.</tt><tt class="descname">select</tt><big>(</big><em>rlist</em>, <em>wlist</em>, <em>xlist</em><span class="optional">[</span>, <em>timeout</em><span class="optional">]</span><big>)</big><a class="headerlink" href="#select.select" title="Permalink to this definition">¶</a></dt>
<dd><p>This is a straightforward interface to the Unix <tt class="xref c c-func docutils literal"><span class="pre">select()</span></tt> system call.
The first three arguments are sequences of ‘waitable objects’: either
integers representing file descriptors or objects with a parameterless method
named <tt class="xref py py-meth docutils literal"><span class="pre">fileno()</span></tt> returning such an integer:</p>
<ul class="simple">
<li><em>rlist</em>: wait until ready for reading</li>
<li><em>wlist</em>: wait until ready for writing</li>
<li><em>xlist</em>: wait for an “exceptional condition” (see the manual page for what
your system considers such a condition)</li>
</ul>
<p>Empty sequences are allowed, but acceptance of three empty sequences is
platform-dependent. (It is known to work on Unix but not on Windows.) The
optional <em>timeout</em> argument specifies a time-out as a floating point number
in seconds. When the <em>timeout</em> argument is omitted the function blocks until
at least one file descriptor is ready. A time-out value of zero specifies a
poll and never blocks.</p>
<p>The return value is a triple of lists of objects that are ready: subsets of the
first three arguments. When the time-out is reached without a file descriptor
becoming ready, three empty lists are returned.</p>
<p id="index-0">Among the acceptable object types in the sequences are Python file objects (e.g.
<tt class="docutils literal"><span class="pre">sys.stdin</span></tt>, or objects returned by <a class="reference internal" href="functions.html#open" title="open"><tt class="xref py py-func docutils literal"><span class="pre">open()</span></tt></a> or <a class="reference internal" href="os.html#os.popen" title="os.popen"><tt class="xref py py-func docutils literal"><span class="pre">os.popen()</span></tt></a>), socket
objects returned by <a class="reference internal" href="socket.html#socket.socket" title="socket.socket"><tt class="xref py py-func docutils literal"><span class="pre">socket.socket()</span></tt></a>. You may also define a <em class="dfn">wrapper</em>
class yourself, as long as it has an appropriate <tt class="xref py py-meth docutils literal"><span class="pre">fileno()</span></tt> method (that
really returns a file descriptor, not just a random integer).</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last" id="index-1">File objects on Windows are not acceptable, but sockets are. On Windows,
the underlying <tt class="xref c c-func docutils literal"><span class="pre">select()</span></tt> function is provided by the WinSock
library, and does not handle file descriptors that don’t originate from
WinSock.</p>
</div>
</dd></dl>
<dl class="attribute">
<dt id="select.select.PIPE_BUF">
<tt class="descclassname">select.</tt><tt class="descname">PIPE_BUF</tt><a class="headerlink" href="#select.select.PIPE_BUF" title="Permalink to this definition">¶</a></dt>
<dd><p>Files reported as ready for writing by <a class="reference internal" href="#module-select" title="select: Wait for I/O completion on multiple streams."><tt class="xref py py-func docutils literal"><span class="pre">select()</span></tt></a>, <a class="reference internal" href="#select.poll" title="select.poll"><tt class="xref py py-func docutils literal"><span class="pre">poll()</span></tt></a> or
similar interfaces in this module are guaranteed to not block on a write
of up to <a class="reference internal" href="#select.select.PIPE_BUF" title="select.select.PIPE_BUF"><tt class="xref py py-const docutils literal"><span class="pre">PIPE_BUF</span></tt></a> bytes.
This value is guaranteed by POSIX to be at least 512. Availability: Unix.</p>
<p class="versionadded">
<span class="versionmodified">New in version 2.7.</span></p>
</dd></dl>
<div class="section" id="edge-and-level-trigger-polling-epoll-objects">
<span id="epoll-objects"></span><h2>16.1.1. Edge and Level Trigger Polling (epoll) Objects<a class="headerlink" href="#edge-and-level-trigger-polling-epoll-objects" title="Permalink to this headline">¶</a></h2>
<blockquote>
<div><p><a class="reference external" href="http://linux.die.net/man/4/epoll">http://linux.die.net/man/4/epoll</a></p>
<p><em>eventmask</em></p>
<table border="1" class="docutils">
<colgroup>
<col width="33%" />
<col width="67%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Constant</th>
<th class="head">Meaning</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">EPOLLIN</span></tt></td>
<td>Available for read</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">EPOLLOUT</span></tt></td>
<td>Available for write</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">EPOLLPRI</span></tt></td>
<td>Urgent data for read</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">EPOLLERR</span></tt></td>
<td>Error condition happened on the assoc. fd</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">EPOLLHUP</span></tt></td>
<td>Hang up happened on the assoc. fd</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">EPOLLET</span></tt></td>
<td>Set Edge Trigger behavior, the default is
Level Trigger behavior</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">EPOLLONESHOT</span></tt></td>
<td>Set one-shot behavior. After one event is
pulled out, the fd is internally disabled</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">EPOLLRDNORM</span></tt></td>
<td>Equivalent to <tt class="xref py py-const docutils literal"><span class="pre">EPOLLIN</span></tt></td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">EPOLLRDBAND</span></tt></td>
<td>Priority data band can be read.</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">EPOLLWRNORM</span></tt></td>
<td>Equivalent to <tt class="xref py py-const docutils literal"><span class="pre">EPOLLOUT</span></tt></td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">EPOLLWRBAND</span></tt></td>
<td>Priority data may be written.</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">EPOLLMSG</span></tt></td>
<td>Ignored.</td>
</tr>
</tbody>
</table>
</div></blockquote>
<dl class="method">
<dt id="select.epoll.close">
<tt class="descclassname">epoll.</tt><tt class="descname">close</tt><big>(</big><big>)</big><a class="headerlink" href="#select.epoll.close" title="Permalink to this definition">¶</a></dt>
<dd><p>Close the control file descriptor of the epoll object.</p>
</dd></dl>
<dl class="method">
<dt id="select.epoll.fileno">
<tt class="descclassname">epoll.</tt><tt class="descname">fileno</tt><big>(</big><big>)</big><a class="headerlink" href="#select.epoll.fileno" title="Permalink to this definition">¶</a></dt>
<dd><p>Return the file descriptor number of the control fd.</p>
</dd></dl>
<dl class="method">
<dt id="select.epoll.fromfd">
<tt class="descclassname">epoll.</tt><tt class="descname">fromfd</tt><big>(</big><em>fd</em><big>)</big><a class="headerlink" href="#select.epoll.fromfd" title="Permalink to this definition">¶</a></dt>
<dd><p>Create an epoll object from a given file descriptor.</p>
</dd></dl>
<dl class="method">
<dt id="select.epoll.register">
<tt class="descclassname">epoll.</tt><tt class="descname">register</tt><big>(</big><em>fd</em><span class="optional">[</span>, <em>eventmask</em><span class="optional">]</span><big>)</big><a class="headerlink" href="#select.epoll.register" title="Permalink to this definition">¶</a></dt>
<dd><p>Register a fd descriptor with the epoll object.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">Registering a file descriptor that’s already registered raises an
IOError – contrary to <a class="reference internal" href="#poll-objects"><em>Polling Objects</em></a>‘s register.</p>
</div>
</dd></dl>
<dl class="method">
<dt id="select.epoll.modify">
<tt class="descclassname">epoll.</tt><tt class="descname">modify</tt><big>(</big><em>fd</em>, <em>eventmask</em><big>)</big><a class="headerlink" href="#select.epoll.modify" title="Permalink to this definition">¶</a></dt>
<dd><p>Modify a register file descriptor.</p>
</dd></dl>
<dl class="method">
<dt id="select.epoll.unregister">
<tt class="descclassname">epoll.</tt><tt class="descname">unregister</tt><big>(</big><em>fd</em><big>)</big><a class="headerlink" href="#select.epoll.unregister" title="Permalink to this definition">¶</a></dt>
<dd><p>Remove a registered file descriptor from the epoll object.</p>
</dd></dl>
<dl class="method">
<dt id="select.epoll.poll">
<tt class="descclassname">epoll.</tt><tt class="descname">poll</tt><big>(</big><span class="optional">[</span><em>timeout=-1</em><span class="optional">[</span>, <em>maxevents=-1</em><span class="optional">]</span><span class="optional">]</span><big>)</big><a class="headerlink" href="#select.epoll.poll" title="Permalink to this definition">¶</a></dt>
<dd><p>Wait for events. timeout in seconds (float)</p>
</dd></dl>
</div>
<div class="section" id="polling-objects">
<span id="poll-objects"></span><h2>16.1.2. Polling Objects<a class="headerlink" href="#polling-objects" title="Permalink to this headline">¶</a></h2>
<p>The <tt class="xref c c-func docutils literal"><span class="pre">poll()</span></tt> system call, supported on most Unix systems, provides better
scalability for network servers that service many, many clients at the same
time. <tt class="xref c c-func docutils literal"><span class="pre">poll()</span></tt> scales better because the system call only requires listing
the file descriptors of interest, while <tt class="xref c c-func docutils literal"><span class="pre">select()</span></tt> builds a bitmap, turns
on bits for the fds of interest, and then afterward the whole bitmap has to be
linearly scanned again. <tt class="xref c c-func docutils literal"><span class="pre">select()</span></tt> is O(highest file descriptor), while
<tt class="xref c c-func docutils literal"><span class="pre">poll()</span></tt> is O(number of file descriptors).</p>
<dl class="method">
<dt id="select.poll.register">
<tt class="descclassname">poll.</tt><tt class="descname">register</tt><big>(</big><em>fd</em><span class="optional">[</span>, <em>eventmask</em><span class="optional">]</span><big>)</big><a class="headerlink" href="#select.poll.register" title="Permalink to this definition">¶</a></dt>
<dd><p>Register a file descriptor with the polling object. Future calls to the
<a class="reference internal" href="#select.poll" title="select.poll"><tt class="xref py py-meth docutils literal"><span class="pre">poll()</span></tt></a> method will then check whether the file descriptor has any pending
I/O events. <em>fd</em> can be either an integer, or an object with a <tt class="xref py py-meth docutils literal"><span class="pre">fileno()</span></tt>
method that returns an integer. File objects implement <tt class="xref py py-meth docutils literal"><span class="pre">fileno()</span></tt>, so they
can also be used as the argument.</p>
<p><em>eventmask</em> is an optional bitmask describing the type of events you want to
check for, and can be a combination of the constants <tt class="xref py py-const docutils literal"><span class="pre">POLLIN</span></tt>,
<tt class="xref py py-const docutils literal"><span class="pre">POLLPRI</span></tt>, and <tt class="xref py py-const docutils literal"><span class="pre">POLLOUT</span></tt>, described in the table below. If not
specified, the default value used will check for all 3 types of events.</p>
<table border="1" class="docutils">
<colgroup>
<col width="31%" />
<col width="69%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Constant</th>
<th class="head">Meaning</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">POLLIN</span></tt></td>
<td>There is data to read</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">POLLPRI</span></tt></td>
<td>There is urgent data to read</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">POLLOUT</span></tt></td>
<td>Ready for output: writing will not block</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">POLLERR</span></tt></td>
<td>Error condition of some sort</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">POLLHUP</span></tt></td>
<td>Hung up</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">POLLNVAL</span></tt></td>
<td>Invalid request: descriptor not open</td>
</tr>
</tbody>
</table>
<p>Registering a file descriptor that’s already registered is not an error, and has
the same effect as registering the descriptor exactly once.</p>
</dd></dl>
<dl class="method">
<dt id="select.poll.modify">
<tt class="descclassname">poll.</tt><tt class="descname">modify</tt><big>(</big><em>fd</em>, <em>eventmask</em><big>)</big><a class="headerlink" href="#select.poll.modify" title="Permalink to this definition">¶</a></dt>
<dd><p>Modifies an already registered fd. This has the same effect as
<tt class="docutils literal"><span class="pre">register(fd,</span> <span class="pre">eventmask)</span></tt>. Attempting to modify a file descriptor
that was never registered causes an <a class="reference internal" href="exceptions.html#exceptions.IOError" title="exceptions.IOError"><tt class="xref py py-exc docutils literal"><span class="pre">IOError</span></tt></a> exception with errno
<tt class="xref py py-const docutils literal"><span class="pre">ENOENT</span></tt> to be raised.</p>
<p class="versionadded">
<span class="versionmodified">New in version 2.6.</span></p>
</dd></dl>
<dl class="method">
<dt id="select.poll.unregister">
<tt class="descclassname">poll.</tt><tt class="descname">unregister</tt><big>(</big><em>fd</em><big>)</big><a class="headerlink" href="#select.poll.unregister" title="Permalink to this definition">¶</a></dt>
<dd><p>Remove a file descriptor being tracked by a polling object. Just like the
<a class="reference internal" href="#select.poll.register" title="select.poll.register"><tt class="xref py py-meth docutils literal"><span class="pre">register()</span></tt></a> method, <em>fd</em> can be an integer or an object with a
<tt class="xref py py-meth docutils literal"><span class="pre">fileno()</span></tt> method that returns an integer.</p>
<p>Attempting to remove a file descriptor that was never registered causes a
<a class="reference internal" href="exceptions.html#exceptions.KeyError" title="exceptions.KeyError"><tt class="xref py py-exc docutils literal"><span class="pre">KeyError</span></tt></a> exception to be raised.</p>
</dd></dl>
<dl class="method">
<dt id="select.poll.poll">
<tt class="descclassname">poll.</tt><tt class="descname">poll</tt><big>(</big><span class="optional">[</span><em>timeout</em><span class="optional">]</span><big>)</big><a class="headerlink" href="#select.poll.poll" title="Permalink to this definition">¶</a></dt>
<dd><p>Polls the set of registered file descriptors, and returns a possibly-empty list
containing <tt class="docutils literal"><span class="pre">(fd,</span> <span class="pre">event)</span></tt> 2-tuples for the descriptors that have events or
errors to report. <em>fd</em> is the file descriptor, and <em>event</em> is a bitmask with
bits set for the reported events for that descriptor — <tt class="xref py py-const docutils literal"><span class="pre">POLLIN</span></tt> for
waiting input, <tt class="xref py py-const docutils literal"><span class="pre">POLLOUT</span></tt> to indicate that the descriptor can be written
to, and so forth. An empty list indicates that the call timed out and no file
descriptors had any events to report. If <em>timeout</em> is given, it specifies the
length of time in milliseconds which the system will wait for events before
returning. If <em>timeout</em> is omitted, negative, or <a class="reference internal" href="constants.html#None" title="None"><tt class="xref py py-const docutils literal"><span class="pre">None</span></tt></a>, the call will
block until there is an event for this poll object.</p>
</dd></dl>
</div>
<div class="section" id="kqueue-objects">
<span id="id1"></span><h2>16.1.3. Kqueue Objects<a class="headerlink" href="#kqueue-objects" title="Permalink to this headline">¶</a></h2>
<dl class="method">
<dt id="select.kqueue.close">
<tt class="descclassname">kqueue.</tt><tt class="descname">close</tt><big>(</big><big>)</big><a class="headerlink" href="#select.kqueue.close" title="Permalink to this definition">¶</a></dt>
<dd><p>Close the control file descriptor of the kqueue object.</p>
</dd></dl>
<dl class="method">
<dt id="select.kqueue.fileno">
<tt class="descclassname">kqueue.</tt><tt class="descname">fileno</tt><big>(</big><big>)</big><a class="headerlink" href="#select.kqueue.fileno" title="Permalink to this definition">¶</a></dt>
<dd><p>Return the file descriptor number of the control fd.</p>
</dd></dl>
<dl class="method">
<dt id="select.kqueue.fromfd">
<tt class="descclassname">kqueue.</tt><tt class="descname">fromfd</tt><big>(</big><em>fd</em><big>)</big><a class="headerlink" href="#select.kqueue.fromfd" title="Permalink to this definition">¶</a></dt>
<dd><p>Create a kqueue object from a given file descriptor.</p>
</dd></dl>
<dl class="method">
<dt id="select.kqueue.control">
<tt class="descclassname">kqueue.</tt><tt class="descname">control</tt><big>(</big><em>changelist</em>, <em>max_events</em><span class="optional">[</span>, <em>timeout=None</em><span class="optional">]</span><big>)</big> → eventlist<a class="headerlink" href="#select.kqueue.control" title="Permalink to this definition">¶</a></dt>
<dd><p>Low level interface to kevent</p>
<ul class="simple">
<li>changelist must be an iterable of kevent object or None</li>
<li>max_events must be 0 or a positive integer</li>
<li>timeout in seconds (floats possible)</li>
</ul>
</dd></dl>
</div>
<div class="section" id="kevent-objects">
<span id="id2"></span><h2>16.1.4. Kevent Objects<a class="headerlink" href="#kevent-objects" title="Permalink to this headline">¶</a></h2>
<p><a class="reference external" href="http://www.freebsd.org/cgi/man.cgi?query=kqueue&sektion=2">http://www.freebsd.org/cgi/man.cgi?query=kqueue&sektion=2</a></p>
<dl class="attribute">
<dt id="select.kevent.ident">
<tt class="descclassname">kevent.</tt><tt class="descname">ident</tt><a class="headerlink" href="#select.kevent.ident" title="Permalink to this definition">¶</a></dt>
<dd><p>Value used to identify the event. The interpretation depends on the filter
but it’s usually the file descriptor. In the constructor ident can either
be an int or an object with a fileno() function. kevent stores the integer
internally.</p>
</dd></dl>
<dl class="attribute">
<dt id="select.kevent.filter">
<tt class="descclassname">kevent.</tt><tt class="descname">filter</tt><a class="headerlink" href="#select.kevent.filter" title="Permalink to this definition">¶</a></dt>
<dd><p>Name of the kernel filter.</p>
<table border="1" class="docutils">
<colgroup>
<col width="38%" />
<col width="63%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Constant</th>
<th class="head">Meaning</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_FILTER_READ</span></tt></td>
<td>Takes a descriptor and returns whenever
there is data available to read</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_FILTER_WRITE</span></tt></td>
<td>Takes a descriptor and returns whenever
there is data available to write</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_FILTER_AIO</span></tt></td>
<td>AIO requests</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_FILTER_VNODE</span></tt></td>
<td>Returns when one or more of the requested
events watched in <em>fflag</em> occurs</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_FILTER_PROC</span></tt></td>
<td>Watch for events on a process id</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_FILTER_NETDEV</span></tt></td>
<td>Watch for events on a network device
[not available on Mac OS X]</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_FILTER_SIGNAL</span></tt></td>
<td>Returns whenever the watched signal is
delivered to the process</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_FILTER_TIMER</span></tt></td>
<td>Establishes an arbitrary timer</td>
</tr>
</tbody>
</table>
</dd></dl>
<dl class="attribute">
<dt id="select.kevent.flags">
<tt class="descclassname">kevent.</tt><tt class="descname">flags</tt><a class="headerlink" href="#select.kevent.flags" title="Permalink to this definition">¶</a></dt>
<dd><p>Filter action.</p>
<table border="1" class="docutils">
<colgroup>
<col width="38%" />
<col width="63%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Constant</th>
<th class="head">Meaning</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_EV_ADD</span></tt></td>
<td>Adds or modifies an event</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_EV_DELETE</span></tt></td>
<td>Removes an event from the queue</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_EV_ENABLE</span></tt></td>
<td>Permitscontrol() to returns the event</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_EV_DISABLE</span></tt></td>
<td>Disablesevent</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_EV_ONESHOT</span></tt></td>
<td>Removes event after first occurrence</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_EV_CLEAR</span></tt></td>
<td>Reset the state after an event is retrieved</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_EV_SYSFLAGS</span></tt></td>
<td>internal event</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_EV_FLAG1</span></tt></td>
<td>internal event</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_EV_EOF</span></tt></td>
<td>Filter specific EOF condition</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_EV_ERROR</span></tt></td>
<td>See return values</td>
</tr>
</tbody>
</table>
</dd></dl>
<dl class="attribute">
<dt id="select.kevent.fflags">
<tt class="descclassname">kevent.</tt><tt class="descname">fflags</tt><a class="headerlink" href="#select.kevent.fflags" title="Permalink to this definition">¶</a></dt>
<dd><p>Filter specific flags.</p>
<p><tt class="xref py py-const docutils literal"><span class="pre">KQ_FILTER_READ</span></tt> and <tt class="xref py py-const docutils literal"><span class="pre">KQ_FILTER_WRITE</span></tt> filter flags:</p>
<table border="1" class="docutils">
<colgroup>
<col width="39%" />
<col width="61%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Constant</th>
<th class="head">Meaning</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_LOWAT</span></tt></td>
<td>low water mark of a socket buffer</td>
</tr>
</tbody>
</table>
<p><tt class="xref py py-const docutils literal"><span class="pre">KQ_FILTER_VNODE</span></tt> filter flags:</p>
<table border="1" class="docutils">
<colgroup>
<col width="39%" />
<col width="61%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Constant</th>
<th class="head">Meaning</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_DELETE</span></tt></td>
<td><em>unlink()</em> was called</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_WRITE</span></tt></td>
<td>a write occurred</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_EXTEND</span></tt></td>
<td>the file was extended</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_ATTRIB</span></tt></td>
<td>an attribute was changed</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_LINK</span></tt></td>
<td>the link count has changed</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_RENAME</span></tt></td>
<td>the file was renamed</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_REVOKE</span></tt></td>
<td>access to the file was revoked</td>
</tr>
</tbody>
</table>
<p><tt class="xref py py-const docutils literal"><span class="pre">KQ_FILTER_PROC</span></tt> filter flags:</p>
<table border="1" class="docutils">
<colgroup>
<col width="39%" />
<col width="61%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Constant</th>
<th class="head">Meaning</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_EXIT</span></tt></td>
<td>the process has exited</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_FORK</span></tt></td>
<td>the process has called <em>fork()</em></td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_EXEC</span></tt></td>
<td>the process has executed a new process</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_PCTRLMASK</span></tt></td>
<td>internal filter flag</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_PDATAMASK</span></tt></td>
<td>internal filter flag</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_TRACK</span></tt></td>
<td>follow a process across <em>fork()</em></td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_CHILD</span></tt></td>
<td>returned on the child process for
<em>NOTE_TRACK</em></td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_TRACKERR</span></tt></td>
<td>unable to attach to a child</td>
</tr>
</tbody>
</table>
<p><tt class="xref py py-const docutils literal"><span class="pre">KQ_FILTER_NETDEV</span></tt> filter flags (not available on Mac OS X):</p>
<table border="1" class="docutils">
<colgroup>
<col width="39%" />
<col width="61%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Constant</th>
<th class="head">Meaning</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_LINKUP</span></tt></td>
<td>link is up</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_LINKDOWN</span></tt></td>
<td>link is down</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_LINKINV</span></tt></td>
<td>link state is invalid</td>
</tr>
</tbody>
</table>
</dd></dl>
<dl class="attribute">
<dt id="select.kevent.data">
<tt class="descclassname">kevent.</tt><tt class="descname">data</tt><a class="headerlink" href="#select.kevent.data" title="Permalink to this definition">¶</a></dt>
<dd><p>Filter specific data.</p>
</dd></dl>
<dl class="attribute">
<dt id="select.kevent.udata">
<tt class="descclassname">kevent.</tt><tt class="descname">udata</tt><a class="headerlink" href="#select.kevent.udata" title="Permalink to this definition">¶</a></dt>
<dd><p>User defined value.</p>
</dd></dl>
</div>
</div>
</div>
</div>
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<h3><a href="../contents.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">16.1. <tt class="docutils literal"><span class="pre">select</span></tt> — Waiting for I/O completion</a><ul>
<li><a class="reference internal" href="#edge-and-level-trigger-polling-epoll-objects">16.1.1. Edge and Level Trigger Polling (epoll) Objects</a></li>
<li><a class="reference internal" href="#polling-objects">16.1.2. Polling Objects</a></li>
<li><a class="reference internal" href="#kqueue-objects">16.1.3. Kqueue Objects</a></li>
<li><a class="reference internal" href="#kevent-objects">16.1.4. Kevent Objects</a></li>
</ul>
</li>
</ul>
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